Light measuring apparatus



Dec. 17, 1940.

M. w. SCHELDORF LIGHT MEASURING APPARATUS Filed May 26,

Fig.1;

WATT HOUR METER Marvel W. Sahel hf" by His ttohne y.

Patented Dec. 17, 1940 UNITED STATES LIGHT MEASURING APPARATUS Marvel W.Scheldorf, Schenectady, N. Y., assignor to General Electric Company, acorporation of New York Application May 26, 1938, Serial No. 210,247

2 Claims.

My invention relates to apparatus for measuring light and particularlyto such apparatus which includes a photo-electric device havingcapacitance and variable resistance and which is used for measuringsubstantially constant or slowly varying radiation. It is the object ofmy invention to provide an improved apparatus for making suchmeasurements which apparatus, while including a photo-electric device ofthe same form as employed as before, enables one to avoid certaindifliculties heretofore encountered. A further object is the provisionof such apparatus which is simple in construction, accurate and reliablein operation and inexpensive to manufacture.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

Referring to the drawing, Fig. 1 is a circuit diagram illustrating oneembodiment of my invention and Fig. 2 is a detail thereof showing amodification.

In the drawing, I have shown at I a photoelectric tube which is arrangedto receive radiation from a source 2,,such for example as a mercuryvapor lamp which is shown connected to be energized from the source ofdirect current 3. Inasmuch as the radiation received from the source 2is substantially constant or varies only slowly, Ienergize thephoto-electric tube by alternating current and I amplify the responsemade thereby by means of the alternating cur-.

rent amplifier 4 whose output circuit connects with the measuringdevice. The amplifier which I employ is of well known form and isrepresented as comprising two capacity coupled stages including theelectron discharge devices 5, 6. The amplifier may be supplied withdirect current from any suitable source. In the drawing, I have shown itsupplied through the voltage divider I from the full wave rectifier 8which may be vof any suitable well known form, the

same being supplied from the source 9 of alternating current through thetransformer III'. Theinput circuit of the amplifier 4 connects throughthe capacitor I2 with the opposite ends of the resistor I3 whereby itwill amplify whatever alternating current traverses that resistor.

The resistor I 3 and the section l5 of the divider I are included inwhat may be termed a direct current circuit upon the current of whichthere is superposed an alternating current whose amplitude depends uponthe intensity of the radiation received by the photo-electric tube. Thealternating current is supplied by the secondary winding l6, of thetransformer I0, the voltage of which winding is low whereby the maximumvalue of the voltage applied to the tube corresponds to a point on itscurrent-voltage characteristic which is below the knee thereof. Themid-point of the winding l6 connects through the conductor I 'I with thelower end of the section I5 of the divider. One end of the secondary I6connects through the photo-electric tube I with the upper end of theresistor I3 whose lower end connects with the upper end of the sectionI5. The other end of the secondary I6 connects through the balancingmeans I8 also with the upper end of the resistor I3. Thus, the so-calleddirect current circuit comprising the source of direct current I5 andthe resistor I3 has in effect two branches, the one branch includingone-half of the winding I6 and the photo-electric tube I and the otherbranch including the other half of the secondary lfiand the balancingmeans I8. It will also beseen that the secondary I6, the photo-electrictube I, and the balancing means I8 form in themselves a closed seriescircuit for the alternating current induced in the secondary l6.

The balancing means I8 comprises the resistor R and the capacitor Cwhich, in Fig. 1, are shown arranged in parallel. The resistor is madesubstantially equal in value to the .dark resistance of thephoto-electric tube. capacitor C is made substantially equal in value tothe capacitance of the photo-electric tube. The capacitance and the darkresistance of the photo-electric tube I are therefore balanced with theresult that when no radiation reaches 'the tube no alternating currentis superposed upon the direct current passing the resistor I3 and henceno alternating voltage is applied to the input circuit of the amplifier.For the purpose of equalizing the capacitances of the two branchcircuits, I preferably employ an adjustable capacitor C whereby afterthe apparatus is set up this capacitor may be carefully adjusted so thatno response is made in the output circuit of the amplifier due to anydifference in capacitance of the two branches. Any suitable alternatingcurrent responsive device may be employed in the output circuit of theamplifier by which the radiation received by the photoelectric tube maybe indicated or measured. 1 have chosen to illustrate such an indicatingor measuring device as comprising the watt hour meter 22 whose currentwinding connects through the conductors 23 and the transformer 24 withthe output circuit of the amplifier and whose voltage winding connectsthrough the conductors 25 with the source of alternating current 9.

The operation of the apparatus will be readily apparent from the abovedescription; Whenv there is no radiation being received by thephotoelectric tube I, there is no alternating current applied to theresistor I3 since the resistance andthe capacitance of the tube arecompletely balanced respectively by the resistor R and the The anysuitable integrated light units such as.

lumen-hours or, if desired, the meter may be calibrated to read in suchunits.

While I have shown the balancing means l8 as comprising the resistor andthe capacitor arranged in parallel, these elements may, it desired, bearranged in series as shown, for example, in

the modified form illustrated by Fig. 2. If the series arrangement isemployed the resistor and capacitor will have somewhat different valuesthan in the parallel arrangement. For example, if in the seriesarrangement the resistorbe denoted by R1 and the capacitor be denoted byC1 then the value of the resistor R1 in terms of the resistor R will begiven by the equation:

i where and The modification comprising Fig. 2 also shows that thephoto-electric tube and the balancing means therefor may be suppliedwith alternating current by connecting them with the ends of a resistor,shown at 2', included in an alternating current circuit supplied fromthe source 9, the

conductor 11 being connected with the mid-point oi the resistor.

I have chosen the particular embodiments described above as illustrativeof my invention and it'will be apparent that various other modificationsmay be made without departing from the spirit and scope of my inventionwhich modifications I aim to cover by the appended claims.

What I claim as new and desire to secure by Letters Patent oi the UnitedStates is:

1. Radiation measuring apparatus comprising a photoelectric device,balancing means thereior having constant resistance and capacitance anda source of alternating current having an intermediate point alltogether in a series circuit, a source of direct current and animpedance con heated in series between said point and a point of saidcircuit between said device and said balancing means and measuring meansresponsive to the alternating current superposed on the direct currentin said impedance.

2. Radiation measuring apparatus comprising a photoelectric tube,balancing means therefor and a source of alternating current having amidpoint all connected together in a series circuit, said balancingmeans comprising a resistor whose resistance is substantially equal tothe no-radiation resistance of said tube and a capacitor havingsubstantially the same capacitance as said tube, a source of directcurrent and an impedance connected in series between said point and apoint of said series circuit between said tube and said balancing meanswhereby the direct current in said impedance has an alternating currentsuperposed on it proportional to the radiation received by said deviceand a meter responsive to said superposed alternating current.

MARVEL W. SCHELDORF.

